Just discovered this by accident. While in Preview.app when you when you slide three fingers down on your track pad you can get a preview of the top most recent documents opened in a Cover Flow-like menu along the bottom go the screen.

[crarko adds: This is another of those things I wasn't able to reproduce with the Magic Trackpad, so I don't now if this is the result of some setting or perhaps a third-party utility I don't have. Try and see if you can make this work, and please post your results in the comments if you do.]

Moreover, if you hover over a document based app's dock icon and do the three-finger-down swipe, it will give you the display for that app regardless of which app is in the foreground or whether the app is running. (This is a reason to keep non-running apps in the dock.)